2.0 Analysis 2.1 Introduction The pilot was experienced and qualified for the flight, in accordance with the existing regulations. The weather was not a contributing factor in the occurrence. The helicopter systems were examined to the degree possible, and no evidence of a malfunction contributing to the occurrence was found, except for the failed STB in the main rotor head. Therefore, the investigation will focus on the bearing failure, the loss of control, and why the helicopter was considered to be airworthy despite the fact that the blisters found on the STBs were beyond the manufacturer's specifications. 2.2 The Spherical Thrust Bearing Debonding Failure There may be a misconception among some AS350 helicopter operators that the 6,400-hour life of an STB, as originally stated by the manufacturer, is for the whole bearing. However, the applicable MSR revision states that the rubber portions of the STBs are on condition and are to be inspected daily, usually during the ALF inspection. Although the AME had noted that one STB was blistered, he did not follow the STB inspection/rejection criteria specified by the manufacturer, and he released the helicopter for service. Because the blisters were not measured, it is not known whether the condition of the STB exceeded the manufacturer's rejection criteria. STBs fail in a progressive manner, as rubber gradually squeezes out from between the metal plates and blisters grow. The dimensions of the blisters prior to the ferry flight are not known; however, since the STB failed 4.7 flight hours after it was last inspected on the morning of the occurrence, prior to the day's flights, the condition of the STB at the time of the inspection must have exceeded the manufacturer's inspection/rejection criteria. Moreover, the blisters would have grown since the time that they were first noted by the AME, 81 flight hours prior to the occurrence, and they may have exceeded the manufacturer's inspection/rejection criteria during that period. Nonetheless, the helicopter was released for service until the STB could be changed at the next 500-hour maintenance inspection, which was scheduled at 4,580.8 hours, 179.6 flight hours after the occurrence. Although the manufacturer had sent an urgent fleet telex on the subject of STB inspections to all AS350 and AS355 operators, the content of the telex was not included in the flight manual. Furthermore, the circulation of the telex within the company to employees other than maintenance personnel was not determined. Pilots may not have been alerted to the consequences of STB degradation and the accompanying sudden increase in main rotor vibrations. The pilot was not aware that sudden, abnormal main rotor vibration levels are indicative of an impending STB failure. As a result, when the vibration level increased during the ferry flight, the pilot believed that the main rotor was out-of-track, rather than suspecting that he was experiencing an STB failure, and he continued the flight. As the flight progressed, the STB failed in fatigue; the vibrations increased as the rubber squeezed out and, finally, the elastomeric section debonded and departed the rotor head. The balance of the whole main rotor head was now upset to the extent that the pilot was unable to control the helicopter. 3.0 Conclusions 3.1 Findings There may be a misconception shared by some AS350 operators that the 6,400-hour service life for the metal portion of the bearing also applies to the rubber portion. The rubber portion of the bearing is not a life item, but rather an on condition item. Although the AME had noted that one STB was blistered, he did not follow the STB inspection/rejection criteria specified by the manufacturer, and he released the helicopter for service. The condition of the STB was beyond the manufacturer's specifications when the aircraft was dispatched on the maintenance ferry flight. An STB failure results in a C of G shift of the main rotor head, and results in a severe one-per-revolution vibration. There is no warning or corrective action listed in the flight manual regarding sudden increases in vibration levels and addressing the fact that they could be caused by the failure of an STB. The manufacturer advised the AS350 and AS355 operators of this bearing problem in 1991 and again in 1992. As the circulation of the telex within the company to employees other than maintenance personnel was not determined and as the information in the telex is not in the flight manual, pilots may not have been alerted to the consequences of deteriorating STBs and the accompanying increase in main rotor vibrations. The pilot was unaware of the significance of increasing vibration levels and, believing that there was a main rotor blade out-of-track condition, he continued the flight. The red blade STB on the main rotor head failed and caused a loss of control. The pilot's seat broke free from the helicopter at impact. The shoulder harness and inertia reel, which were attached to the back of the seat, became ineffective when the seat broke free. The pilot's helmet was cracked in three places; it came off the pilot's head during the crash sequence because the chin strap was not fastened. 3.2 Causes The main rotor vibration levels increased and control of the helicopter was lost when one of three spherical thrust bearings (STB) on the main rotor head failed in fatigue. Although it had been noted that an STB was blistered, the manufacturer's inspection/rejection criteria were not followed, and the helicopter was allowed to remain in service. 4.0 Safety Action 4.1 Action Taken Transport Canada (TC) featured this occurrence in the Aviation Safety Vortex, issue 6/94, and also in the Aviation Safety Maintainer, issue 4/94. These TC publications are sent to all licensed personnel in the helicopter community. The articles highlighted the requirement to ensure the serviceability of the spherical thrust bearings (STBs). In addition, a TSB Aviation Safety Advisory was forwarded to TC indicating that the aircraft flight manual should include a warning or caution that abnormal vibration levels can be caused by degradation of STBs.